(1) Field of the Invention
The present invention relates to an echo canceller for data transmission, and more specifically to an echo canceller which generates a replica of echo from the sending data in case the sending data appears on the receive line on the occasion of connecting the transmit and receive lines to a 2-wire lead through a hybrid circuit such as a 2-wire/4-wire interface used in a telephone system.
(2) Description of the Prior Art
An echo canceller of the prior art has been required to have a number of taps of a transversal filter to from a replica of echo for echo cancelling with high accuracy as described in IEEE, Proceedings of GLobe Com' 85 pp 1311-1316). However, since the amount of hardware is proportional to 2.sup.N when the tap length is N, if N=16, for example, the capacity of 66 k words of RAM is required for generating the coefficient. Namely, when one word is composed of at least 10 bits, a total of 660 kbits are required. Accordingly, it is no longer impossible to load a RAM into a chip even when the 1.3 .mu.m process is used and considerations for simplification of the device has not been taken into account.
In the case where a replica of echo generator having a short impulse response is used, and since residual echo having a sharp rising characteristic is generated when the replica of echo is subtracted from the echo, noise is generated if the residual echo is applied to a line equalizer because the equalizer has a high frequency gain at a high frequency band.